Research conclusions：Combined with a special subway excavation project, this paper analyzes the guard pile stiffness as support system, support stiffness and embedded depth of guard pile for foundation pit with single-factor analysis based on the simulation analysis. It concludes: (1)The deformation of exterior-protected construction can be reduced and the stability of foundation pit can be enhanced through improving the guard pile stiffness and support stiffness. However, if the stiffness is too much, the axial force becomes big, resulting in the support unstable and damaged and increase of the construction cost. (2)The depth of guard pile should be controlled in the range from 1.2 h to 1.4 h. If the embedded depth is less, the deformation of the pile becomes large, resulting in the foundation pit damage. If the embedded depth is too much, the effect on deformation control is not obvious, resulting in increase of the construction cost. (3) After Optimization of the pile diameter, the deformation control can meet the requirement of ground subsidence and the construction cost can be lowered much under the effect of guard pile with diameter of Φ 800.